1. Field of the Invention
The present invention provides a clock circuit for microprocessors. This invention is particularly well-suited for use in Personal Digital Assistants, mobile communication devices, cellular phones, and wireless two-way email communication devices (collectively referred to herein as xe2x80x9cmobile devicesxe2x80x9d). The invention may provide utility, however, in any device that is subjected to high levels of electromagnetic interference.
2. Description of the Related Art
Known clock circuits commonly include crystal oscillators that resonate at a certain frequency. Once the crystal oscillator begins vibrating at its resonant frequency, the resonant frequency is typically maintained by feeding back an in-phase signal from one terminal of the crystal oscillator to the other terminal of the crystal oscillator. This allows the clock circuit to generate a substantially constant clock speed.
Known clock circuits generally have high impedances in order to reduce power consumption. The clock circuit""s high impedance, however, makes it susceptible to interference from strong electromagnetic signal sources. For example, in a mobile device, a transmission circuit may include a power amplifier that interferes with the clock circuit during transmission, temporarily changing the frequency of the oscillator.
Typical mobile devices isolate the clock circuit from the strong electromagnetic interference that occurs during transmission by shielding the clock circuit with an EMI shield. In addition, a typical mobile device may include an external buffer amplifier that protects the integrity of the clock circuit. These known techniques generally add complexity and high component counts to the device. Furthermore, these techniques may require the circuit to restart the crystal oscillator in the event that the oscillation has stopped or slowed as a result of a strong interference signal.
A clock circuit is disclosed which comprises an analog clock element, a digital clock element, and a controller. The analog clock element is configured to generate an oscillating output. The digital clock element is configured to generate a digital clock output. The controller is configured to switch between the analog clock element and the digital clock element. The oscillating output and the digital clock output have substantially equivalent frequencies.
In a first embodiment of the present invention, a clock circuit for a mobile device has a clock circuit output and comprises a state signal having a first state and a second state, wherein the state signal is in the first state while the mobile device is transmitting, a controller coupled to the state signal, the controller being configured to generate a digital clock enabling signal when the state signal is in the first state and to generate an analog clock enabling signal when the state signal is in the second state, an analog clock circuit coupled to the controller to receive the analog clock enabling signal and configured to generate an oscillating output, wherein the oscillating output is enabled as the clock circuit output when the analog clock circuit receives the analog clock enabling signal from the controller, and a digital clock circuit coupled to the controller to receive the digital clock enabling signal and configured to generate a digital clock output having a frequency that is substantially equal to the frequency of the oscillating output, wherein the digital clock output is enabled as the clock circuit output when the digital clock circuit receives the digital clock enabling signal from the controller.
According to a further aspect of the present invention, a clock circuit comprises an analog clock element configured to generate an oscillating output, a digital clock element configured to generate a digital clock output, and a controller configured to switch between the analog clock element and the digital clock element, wherein the oscillating output and the digital clock output have substantially equivalent frequencies.
In a still further embodiment of the invention, a method of isolating a crystal oscillator in a mobile device from electromagnetic interference comprises the steps of receiving a state signal which indicates that generation of an electromagnetic field near the crystal oscillator is anticipated, detecting an edge of a clock circuit signal after the state signal has been received, and enabling a digital clock circuit and disabling the crystal oscillator from the clock circuit when the edge is detected.
A method of maintaining oscillation of a crystal oscillator, according to another aspect of the invention, comprises the steps of detecting an edge of a clock circuit signal, enabling a digital clock circuit when the edge is detected, and driving the crystal oscillator with the digital clock circuit, wherein the output of the digital clock circuit and the crystal have substantially equivalent frequencies.